1. Technical Description
The invention relates to an apparatus and a method for controlling the coating thickness of an optical fiber.
2. Description of the Prior Art
Optical fibers are used, for example, in fiber optic cables. Immediately after their manufacture, the fibers are usually coated with a coating in a continuous process for protecting the surface. The fiber is fabricated by a conventional process, i.e. drawn from a molten preform in a drawing furnace. Because the drawing process is performed at high temperatures (for glass fibers at approximately 2,000.degree. C.), while the coating is typically applied at temperatures below approximately 100.degree. C., the fiber generally passes through a cooling unit placed between the manufacturing unit and the coating unit. Inside the cooling unit, the fiber most commonly moves in a protective gas flow which protects the surface of the fiber from aggressive air constituents and dust particles.
In the coating unit, the fiber passes through a bath containing the frequently pressurized liquid coating material and exits through a coating nozzle. The thickness of the surface coating depends both on the drawing speed of the fiber and the viscosity of the coating material. In addition, the coating unit generally includes a UV lamp system, since preferably UV-Hardenable coating materials are used.
A constant coating thickness is essential for the quality of the fibers and is consequently defined within narrow limits. In a last step, the fiber including the coating passes through a measuring device for measuring the fiber diameter. The measured value is then transmitted to a control element which as a function of the measured diameter controls a control device so that the coating diameter is constant.
The publication EP 619 275 A2 illustrates an apparatus of the aforedescribed type. The control device therein is a temperature controller which is located in the coating unit and adjusts the temperature of the liquid coating material. As a result, the viscosity and correspondingly the quantity of the material exiting the coating nozzle changes. The coating thickness can then be controlled to coincide with a target value.
Disadvantageously, however, the changed viscosity of the coating material also alters its flow characteristics, causing undesirable variations in the properties of the applied coating. This is primarily caused by the change in the material properties of the coating due to shear processes between the different layers of the liquid in the regions of the coating nozzle where the coating material acts as a non-Newtonian fluid if traditional process parameters are used. Moreover, the control speed, the control accuracy and the dimensional tolerances of the adjustable thickness region are inadequate, in particular if the drawing speed of the fiber which is used to adjust the core diameter, is changed over a wide range at the beginning and the end of the fiber manufacturing process. These effects as well as environment conditions lead to variations in the fiber temperature, thereby reducing control over the coating process.